The present invention relates generally to an integrated circuit (IC) inductor, and more particularly, to a symmetric crossover structure of two lines for radio frequency (RF) integrated circuits.
A voltage-controlled oscillator (VCO) in a wireless transceiver typically employs phase-locked loop (PLL) to realize the tunable local oscillator (LO) signal. For the frequency spreading process and temperature variation thereof, the VCO is required to possess wider tunable range. In addition, in wireless communications, smaller silicon area, lower power consumption and lower noise all are desirable for the VCO design.
Recently, several proposed VCO""s gain good noise performance, wider tunable rang and ultra low power consumption by use of bonding wire inductor. However, the bonding wire inductor may be not suitable for integration circuits. Although on-chip inductor is proposed alternatively, for one more inductors are needed for a VCO, it will require large occupied chip area when the on-chip inductor is applied in the VCO. To simultaneously improve the quality factors, such as noise performance, tunable rang and power consumption, and reduce the needed chip area, spiral inductor is proposed for the applications in a differential VCO.
FIG. 1 shows a conventional spiral inductor 10 which includes a conductor line wired in a spiral winding 12 with the most outside line segment 122 connected to an input 14 and the most inside line segment 124 connected to an output 18 by crossing over the spiral winding 12 with a line segment 16 through an higher or lower conductor layer. However, this inductor 10 is not suitable for a differential VCO due to its asymmetric device structure.
FIG. 2 shows a symmetric spiral inductor 20 which includes a spiral winding 22 and a crossover structure 24 composed of two lines 242 and 244 crossing over each other. Even this inductor 20 has its left and right half portions symmetric to the center line 26, the crossover structure 24 still has asymmetric factor. In particular, as shown in FIG. 3, two lines in a same conductor layer must have one of them, e.g., that one denoted by numeral 242, to cross over the other one 244 by jumping to either a lower or higher conductor layer at where they meet with each other, and as a result, high-frequency parasitic capacitors resulted from these two lines 242 and 244 to the substrate containing these two lines 242 and 244 are different due to their arrangement in different-level conductor layers, which then results in obviously asymmetric performance in the crossover structure 24 when such device is operated with high frequency.
In the balanced planar transformer disclosed in U.S. Pat. No. 4,816,784 issued to Rabjohn et al., two spiral inductors are formed by two crossover lines that are symmetric to the center thereof, while it is still asymmetric at the crossover portion of the two lines.
On the other hand, it is obvious to those skilled in the art that a single-layer spiral inductor is disadvantageous to provide large inductance, and to overcome this shortcoming, dual-layer spiral inductor is proposed. However, for the dual-layer spiral inductor is inherently asymmetric in its device structure thereof, the inductances seen from its input and output are different. To improve the shortcoming of the inductance and the asymmetric device structure for on-chip inductor, in U.S. Pat. No. 6,380,835 issued to Lee a symmetric multi-layer spiral inductor is proposed. However, the crossover portion of this spiral inductor is still asymmetric and its device structure is formed with multi-layer conductors, it is thus introduced of serious parasitic effect.
Therefore, it is desired a two lines inductor which has symmetric crossover structure thereof.
An object of the present invention is to provide a symmetric crossover structure of two lines for RF integrated circuits.
Another object of the present invention is to provide an inductor which has two lines crossover structure and it is symmetric.
In a symmetric crossover structure of two lines for RF integrated circuits, according to the present invention, each of the two lines is branched to two routes when they are crossing over each other, of which the first route of the first line uses a lower conductor layer to cross over the first route of the second line and an higher conductor layer to cross over the second route of the second line, and the second route of the first line uses the higher layer to cross over the first route of the second line and the lower layer to cross over the second route of the second line. As a result, the crossover portion of these two lines has a symmetric structure and thus substantially has parasitic effect in high frequency for these two lines.